The present invention relates to antennas in which radiating portions are constructed of straight lengths of metallic wires, rods, or tubing. A transmission line extending between the antenna and the transmitter or receiver is connected to one extremity of the radiating portion of the antenna. Another feature of these antennas is that the radiating portion is elevated or mounted away from the surface of the antenna support. Thus, the invention is not concerned with antennas in which the radiating portion extends directly out from an extended conducting surface, such as a base-driven whip antenna mounted on the body of an automobile, or a vertical or whip tower antenna extending upwardly from the earth.
Precautions must be taken in order to inhibit the flow of radiating currents on auxiliary apparatus associated with antenna systems, such as on the transmission line connecting the antenna to a transmitter or a receiver, and on any metallic support or mast on which the antenna is mounted. Such radiating currents can severely distort and degrade the desired radiation characteristics of the antenna.
This problem has been described in U.S. Pat. No. 2,184,729 to Bailey, which covers means for correcting problems through the use of what is described as a "tubular counterpoise", now commonly called a "decoupling sleeve". What Bailey described as an antenna-counterpoise system is now called a sleeve dipole antenna, generally driven by a coaxial transmission line extending typically upwardly inside of the inner element of the dipole. Bailey refers to the outer dipole element as antenna 1 and part of the inner element as counterpoise 2, each of which is limited to 1/4 wavelength and they constitute a vertical halfwave system. Bailey points out that the outer sheath 2, a decoupling sleeve, forms with the enclosed pipe a high impedance circuit or anti-resonant circuit. The desired decoupling action which inhibits the flow of current on the outside of the pipe support depends upon the resonance of the decoupling sleeve.
There is no reference in the Bailey Patent to the effects of the geometrical shape of the decoupling sleeve on the radiation characteristics of the antenna. All of Bailey's emphasis is on the prevention of unwanted currents on the structure within the decoupling system. In Bailey, the fact that a properly designed decoupling sleeve can simultaneously provide the desired decoupling and improve the radiation characteristics of the antenna is not suggested. This point is of great importance in the present invention.
The radiation patterns of sleeve dipoles with close fitting sleeves, as provided by Bailey, are seriously degraded in comparison with sleeves having diameters that are 5 times or more greater than the diameter of the support pipes or the coaxial lines therein. A large diameter ratio not only promotes better decoupling at the resonant frequency of the sleeve, but widens the bandwidth over which the decoupling is effective. This also is not indicated by Bailey.
Other pertinent prior art is found in a publication entitled, "Facts About Proper VHF Vertical Antenna Design", written by one of the applicants herein, and published by Advanced Electronic Applications, Seattle, Washington. FIG. 8 in the aforesaid publication illustrates a center driven dipole having a total length of 11/4 wavelengths, 5/8 wavelength on each leg. The coaxial line is brought up to the center of the dipole, where the center conductor excites the upper 5/8 wavelength element. The lower 5/8 element consists of the outside of the outer conductor, down for a distance of 3/8 wavelength from the feed point, and then the outside of a 1/4 wavelength cylindrical decoupling sleeve. The lower 5/8 element terminates at the open end of the outer resonant 1/4 wavelength sleeve. A second cylindrical resonant sleeve is fitted below the first in order to produce additional decoupling. Both of the foregoing sleeves are similar to that provided by Bailey in his patent.
In the foregoing prior art, the length of decoupling sleeves is chosen to be resonant to present high impedance in the path of any current on the outside of the sleeve to inhibit this current from continuing to flow along a supporting pipe below the open mouth of the sleeve or sleeves. The resonant condition for a cylindrical sleeve occurs at very nearly 1/4 wavelength, and therefore, must be appreciably shorter than a conical or tapered sleeve of the present invention which is shown and described in part in the above-identified publication; and referred to as the "The AEA IsoPole.TM. antenna", FIG. 10, pages 15 and 16.